1. Field of the Invention
The present invention relates to an objective lens for use in a microscope of a low magnification, and more particularly to such objective lens for use in a metallurgical microscope utilizing coaxial reflective illumination.
2. Description of the Prior Art
An objective lens for a microscope of low magnification is already disclosed for example in the U.S. Pat. No. 4,027,951 corresponding to the Japanese Patent Publication No. 52568/1982. Said objective lens is composed of three lens groups, of which a first lens group has a converging refractive power as a field lens for achieving a telecentric structure at the object side, namely with the entrance pupil at an infinite position, a second lens group has a diverging refractive power for focal length compensation and satisfactory correction of the Petzval sum, and a third lens group has a converging refractive power for realizing desired magnification and focal length in combination with the second lens group.
However, the above-described conventional structure with mutually separate first and second lens groups inevitably involves many air-to-lens interfaces and is practically unusable because of the low image contrast, since significant flare is generated in the coaxial reflective illumination method utilizing the objective lens itself for guiding the illuminating light.